It's name shows that it is concerned with two things 1st depression and the 2nd is mania (you can look in categories for it's definition and symptoms) this bipolar disorder is also known as manic-depression. This is the less common type of depression which is rare to find in people around you.

The person who is suffering from it, will have behavior in cyclic shape, sometimes he will go into depressive mood and sometimes mania. There will be no fix behavior, it can be explained by the example like a person who is suffering from it when in his depressive mood he will be having all symptoms of depression and when the time passes he goes into manic mood. It is found that in manic mood the person talks really fast, do things very fast, without thinking makes decision, the person who is manic mood may face a lot of embarrassment due to it's weird behavior, it is found that if mania is not treated at time it can lead you into psychotic state in which you are out of touch with the reality e-g Terrorists.